As we know that Demineralized Water (DM Water) is a good insulator so it can strike our mind that why don’t we use DM water for cooling purpose of Transformer in place of Oil?
To find the answer, we comare the properties of DM Water and Transformer Oil.
Both Transformer Oil and Demineralized water have negligible electrical conductivity.
Water will easily vaporize at 100 °C. But oil is having relatively high boiling point. Boiling point of transformer oil is 140 °C.
Thus in case of oil, the winding temperature are allowed to go above 100 °C but if water is used then above 100 °C the cooling will become ineffective as the water will vaporize and will lead to damage to the Transformer.
Thermal conductivity of water is quite superior to that of oil.
Thermal conductivity of water- 0.609 W/mK
Thermal conductivity of oil – 0.137 W/mK
As water is a polar in nature, therefore the Dielectric Strength of water is quite low when compared with Transformer Oil (non-polar). Typically the Dielectric Strength of Transformer oil is in range of 90-100 kV/cm while that of Demineralized Water is in the range of 55-65 kV/cm.